Commercial airliners don’t leave the ground without a pilot and a co-pilot at the controls. They work together as a check-and-balance to ensure that everything from preflight to post-flight goes successfully. This is an added safety measure to minimize the probability of human error related accidents. Driver assistance devises have been introduced into the automotive industry for the same reason. These systems act as an electronic co-pilot for the driver. Since model year 2000, driver assistance devices have been making headways in most makes and models of vehicles sold around the world. Advanced Driver Assistance Systems (ADAS) is a giant step towards autonomous vehicles.
There are five levels of driver assistance systems; 1) driver assistance, 2) partial automation, 3) conditional automation, 4) high automation and 5) complete automation (figure 1). With model year 2019 vehicles tipping the scales at level 3, the bulk of the vehicles presently coming into our shops fall firmly within levels 1 and 2. Realizing this fact, we need to know enough about these systems to ensure that we can address transmission complaints while fully considering the driver assistance systems’ role.
We will take a look at the 2011 Lincoln MKS, All-wheel drive equipped with a 3.5L Ecoboost engine and a 6F55N transmission. While the systems incorporated here are considered entry-level by today’s ADAS standards, this will give you an accurate overview of vehicles you are presently working on.
Components and System Function
With most driver assistance systems, the system is more reliant on the computer programming as opposed to actual control devices. However, sensor feedback is critical for the system operation. In the case of Lincoln MKS, they use steering angle sensor, several radar sensors and a windshield photosensor to provide direct system inputs. Data is received from other sensors and modules via the CAN bus system. Everything is processed at the PCM. All control commands are sent from the PCM.
The Lincoln MKS uses either a Steering Angle Sensor Module (SASM) (figure 2) or a Steering Wheel Rotation Sensor (figure 3). For vehicles equipped with the SASM, there are two different modules available depending on if the vehicle is equipped with the Auto Park function. There will be a switch in the center console at the rear of the shifter assembly for models equipped with this function.
The steering angle sensor is used to provide the PCM with input on the driver’s intended direction of travel. The sensor is positioned directly on the steering column and responds directly to the movement of the steering wheel. The input is calibrated for a zero neutral position that will indicate straight travel. Any movement left or right of the neutral position will be seen in degrees on a scan tool.
The steering angle sensor can cause drivability issues that can be misinterpreted as transmission faults. In the case of sensor signal failure, a fault code will be stored in the PCM. However, if the sensor is giving inaccurate data to the PCM, this may not be detected.
Whenever the Steering Angle Sensor module or the Steering Wheel Rotation Sensor is replaced, you must calibrate the sensor. Failure to calibrate the sensor can cause system malfunctions and/or drivability concerns. Note, if you do not replace the sensor, use your scan tool to verify a “zero degrees” reading when the vehicle is traveling in a straight line. The PID on your scan tool in PCM data labeled “SWA POS”.
A scan tool is not necessary to calibrate the Steering Wheel Rotation Sensor. The system automatically calibrates the sensor when you make several tight circles. It is recommended to at least perform 6 tight circles; alternating for 3 to the left and 3 to the right.
For models equipped with the SASM, calibration is performed manually by aligning the sensor with the steering shaft with the vehicle in a straight ahead position. In most cases, you will not need to worry about this type of sensor unless you had to replace it. However, it is recommended to use a steering wheel lock if you need to separate the steering shaft from the rack while R&R-ing the transmission. Always mark the rack-to-shaft position prior to removal! If you put it back the way you found it, there should be no need for calibration.
The Radar Sensor (Cruise-Control Module)
The Lincoln MKS uses a radar sensor mounted underneath the front bumper cover, inside the Cruise-Control Module (figure 4). It is primarily used for the Adaptive Cruise Control, but it is also used for Auto Parking and Collision Warning. It is also used in part for the Auto High Beam control system. If the sensor malfunctions, the affected systems will be disabled. A “Cruise Not Available” and/or “Collision Warning Not Available” messages may appear in the instrument cluster.
A dirty radar sensor can cause the parking assist audible alarm to activate while waiting in traffic behind another stopped vehicle. It may also activate at speeds below 4 mph intermittently. Snow and ice build-up will have the same effect. Always check and clean the sensors first before attempting repairs or recalibration.
Also, the MKS adaptive cruise control can become inoperative in poor weather. Heavy precipitation, mud puddles, snow and ice build-up can cause the sensor to read incorrectly. The owner’s manual discourages attempting to use the cruise control under these conditions.
Always note damage and recent body repair work in the area of this sensor. You don’t want to be stuck trying to fix a cruise control issue that was present prior to you working on the vehicle.
The Windshield Photosensor
The Windshield Photosensor is used as an input for the Auto High Beam and Rear-View Mirror dimming functions (figure 5). The sensor assembly is mounted on a bracket secured to the windshield. If the windshield is dirty or if the sensor is not properly secured, the Auto High Beam function will switch the high beams on and off at random. Issues can occur if the windshield has been replaced.
If you suspect an issue with the vehicle drivability issue related to the driver assistance systems, here is a diagnostic trick that could be very useful:
Finding a fault Steering Angle Sensor signal can be tricky if a code is not set. You can monitor the “SWA POS” PID in graphing mode while driving or simply turning the steering wheel from left to right. If you are unable to observe any signal issues, you can disable the ABS system (for testing purposes only) by pulling the ABS module power fuse to observe if drivability issues go away. Note: a common occurrence caused by a failing Steering Angle Sensor is a rumble or shudder sensation while cornering at speeds 35mph or slower.
Unexplained no upshift or commanded downshift complaints can be diagnosed in a similar manner. When the ABS system is disabled, the Collision Warning system is disabled too. Note: An abrupt transmission downshift can be commanded when the Collision Warning system is activated.
In summary, once you understand the purpose and general operation of these new systems, you are empowered to approach new vehicles with greater confidence. You are also able to perform necessary repairs without risk of compromising the operation or safety of the vehicle.